It is known that floor panels for forming a floor covering, thus, floor panels which are intended for being provided on an existing supporting floor, can be of different nature. Two important categories of such floor panels are, on the one hand, the so-called laminate floor panels and, on the other hand, the floor panels constructed as composed parquet panels, which latter also are called “engineered wood” panels.
Mostly, laminate floor panels, for the major part of their thickness, consist of a substrate of a one-piece wood fiberboard, more particularly MDF (Medium Density Fiberboard) or HDF (High Density Fiberboard). On the substrate then a top layer is provided, which then either is formed of one or more sheets impregnated with resin, or is formed of a print, whether or not combined with still other layers. When the one or more sheets impregnated with resin are pressed directly on the substrate, this is called Direct Pressure Laminate (DPL). When first a number of impregnated sheets are pressed together and the obtained whole then is attached to a substrate, this is called High Pressure Laminate (HPL). On the underside, a backing layer can be applied, which mostly is intended for offering a balancing effect in order to provide a warping of the floor panels.
Composed parquet panels usually comprise at least a substrate, a wood layer attached thereto and a wood-based backing layer on the underside of the substrate. The wood layer situated at the upper side functions as a top and decorative layer and mostly is fabricated of hardwood. It can be treated further at its upper side, for example, in order to exert an influence on the appearance thereof and/or to improve the surface quality thereof, for example, by means of coloring, applying a wear-resistant and water-tight transparent lacquer, and so on. The wood-based backing layer mostly consists of a one-piece thin layer of a cheap and mostly soft wood species.
Traditionally, the substrate of a composed parquet panel is composed of a plurality of crosswise-directed laths of wood. In oblong panels, it is also know to replace the laths at the short sides by strips of another material, such as HDF (High Density Fiberboard) or multiplex, in order to increase the strength at these edges. Examples thereof are described in DE 101 63 435 and WO 2007/141605. Also, embodiments of composed parquet panels are known, wherein the entire substrate is replaced by a single continuous wood fiberboard, for example, as described in DE 201 21 836.
Possibly, also other layers, whether or not consisting of wood, may be integrated in such composed parquet panel.
Further, also other floor panels are known which do not fall directly in said two categories, which mostly differ from said categories in that particular top layers are applied on the substrate, such as veneer, cork, vinyl, linoleum, stone, carpet and the like. Thus, the substrate then mostly consists of a single board of MDF (Medium Density Fiberboard) or HDF (High Density Fiberboard).
For information, it is noted that, as commonly known, MDF and HDF consist of wood fibers which, together with a binding agent, are pressed in board shape under the influence of heat and pressure. The production is performed by compressing, in a known manner, a mat of resin-treated fibers until a certain density is reached, wherein the resin is cured such that a bond between the fibers is created. Depending on the degree with which the compression is performed, a final board having a lower or higher density is obtained. The density is the value obtained by taking 1 cubic meter of manufactured board material and determining the weight thereof and then expressing this in kg/cubic meter. When mentioning density herein, then this relates to the average density, in consideration of the fact that the density has a varying density gradient across the board. As a product is concerned which is obtained by compression, the density at the exterior sides of the board is higher than in the middle, and such density gradually decreases from the exterior towards the inside.
Although the term MDF sometimes is applied as a broader term for all wood fiberboard which is realized according to the aforementioned principle, in reality indeed there is a difference between MDF and HDF, wherein it is clear that HDF is a board of higher density than MDF. In the literature, sometimes different density values are given for the limits between MDF and HDF are given, however, in general it may be assumed that this is situated at 800 kg/cubic meter, which indeed corresponds to the usual standards in this respect. Specifically for the description and claims following below applies that by MDF, a fiberboard is understood having a density of less than 800 kg/cubic meter, whereas by HDF, a fiberboard is understood having a density, thus, an average density, of at least 800 kg/cubic meter. In practice, HDF mostly has a density of about 850 kg/cubic meter. Fiberboard below 600 kg/cubic meter is called LDF (Low Density Fiberboard).
It is clear that MDF is a more economic product than HDF, considering that for the same board thickness less basic material, thus, in particular wood fibers, is applied. Moreover, during production the fiber mat must be compressed less forcefully, which requires less energy during production.
In the past, it was common to provide floor panels at their edges with a simple tongue and groove and, when being installed, they were glued into each other at their tongue and groove connection. However, since the second half of the nineties, this manner of connecting is almost completely replaced by the use of purely mechanical couplings with coupling parts which, in the coupled condition of two of such floor panels, provide for a coupling in vertical as well as in horizontal direction. It is clear that the integration of coupling parts, which provide a horizontal as well as a vertical locking, in the edges of the floor panels requires that these coupling parts must show sufficient strength, wherein the strength of the material itself, in which the coupling parts are realized, of course plays an important role. Indeed, it is known that such floor panels can expand and shrink under the influence of temperature and moisture. In particular when shrinking, the floor panels of the mostly floatingly installed floor covering are put under a heavy load, for example, when heavy furniture is placed on such floor covering at opposite sides, which furniture, when the floor covering shrinks, actually has to be carried along. Thus, it are in particular those parts which must provide a horizontal locking, which have to be able to withstand large forces. It is also known that in oblong floor panels, the coupling parts at the short sides mostly have to absorb larger forces per length unit than the coupling parts at the long sides.
It is also known that wood fiberboard in general, and more particularly MDF and HDF, is an anisotropic material, wherein tearing may occur very easily in planes parallel to the surface of the board. Thus, with coupling parts performed in such material, care must be taken that certain parts thereof are not damaged during use, for example, by tearing, tearing off, breaking off or the like, which, of course, may result in less good locking or even may mean that there is no more locking at all. Thus, it is also known to reinforce coupling parts of wood fiber material by impregnating the wood fiberboard and/or the surface with a reinforcing agent, such as polyurethane, or providing such agent herein in another manner. A number of examples where modifications are performed in wood fiberboard on the location where a coupling part must be realized, or a coupling part is realized already, are known, amongst others, from WO 02/24421, DE 199 63 203, FIG. 24e or WO 2008/060232. Sometimes, such modifications also are performed in order to realize a seal against moisture penetration at the surface of coupling parts formed in wood fiberboard, for example, as known from WO 2008/078181.